Rapid expansion of the invasive-like red macroalga, Chondria tumulosa (Rhodophyta), on the coral reefs of the Papahānaumokuākea Marine National Monument.

A cryptogenic, invasive-like red macroalga, Chondria tumulosa, was first observed in 2016 forming thick mats on the forereef of Manawai Atoll within Papahānaumokuākea Marine National Monument. Subsequent expeditions revealed an increased abundance of this alga. In 2021, unattached C. tumulosa was observed forming a network of dark, meandering accumulations throughout the atoll's inner lagoon. High-resolution satellite imagery revealed that these accumulations became visible in 2015 (length: ~0.74 km; area: ~0.88 km ) and increased 56-fold in length and 115-fold in area by 2021 (length: 41.32 km; area: 101.34 km ). An exponential expansion rate of ~16.02 km · y (length), ~44.75 km  · y (area). This study presents the comprehensive temporal and spatial expansion of C. tumulosa accumulations for Manawai Atoll since its discovery, providing ecologist and resource managers with a proxy to gauge the overall abundance trend of this invasive-like alga.